The present invention relates to an electric power steering apparatus using a motor as a power source for steering assists.
FIG. 1 shows a schematic cross sectional view of a conventional electric power steering apparatus. The electric power steering apparatus comprises a first steering shaft 2 connected with a steering wheel 1, a second steering shaft 4 connected with the first steering shaft 2 through a torsion bar 3, a torque sensor 5 for detecting the relative rotational displacement between the first and second steering shafts 2, 4, a motor 6 for steering assists, which is driven corresponding to the detected result of the torque sensor 5, and a reduction mechanism 9 having a worm 7 and a worm wheel 8 for reducing and transmitting the output power of the motor 6 to the second steering shaft 4. The operation of the steering mechanism corresponding to the turning of the steering wheel 1 is assisted by the rotation of the motor 6 so as to reduce the driver""s burden during steering.
A worm shaft 10 is connected with the output shaft of the motor 6 and disposed to cross the shaft axis of the second steering shaft 4. The worm 7 is integrally formed
FIG. 2 shows an enlarged cross sectional view of a worm wheel in an electric power steering apparatus disclosed in the Japanese Utility Model Publication No. 2556890. The worm wheel 8 comprises an annular tooth portion 81 and a boss portion 82. The annual tooth portion 81 is made of synthetic resin, and has teeth 8a engaging with the worm 7. The boss portion 82 is made of metal, and is engaging with the annular tooth portion 81 at the inside thereof. This structure allows not only [to reduce] the noise at the engaging of the worm wheel 8 and the worm 7 to be reduced, but also the processability of the teeth to be improved. The annular tooth portion 81 is made of nylon synthetic resin material mixed with glass fibers.
The glass fibers mixed in the nylon synthetic material, used in making the annular tooth portion of the conventional worm wheel, has an aspect ratio (ratio of the length to the diameter) of less than 100. Due to the small aspect ratio, the fatigue strength of the teeth projecting from the peripheral of the annular tooth portion and that of the root portion of the teeth was not sufficient. Thus, the improvement of the fatigue strength of the teeth and the root portion of the teeth were desired. More precisely speaking, the annular tooth portion is formed by injection molding, in which nylon synthetic material is injected into a mould by an injection screw. And at the injection, the glass fibers mixed into the synthetic resin material run along the stream of the synthetic resin material into the mould. As a result, the directions of the fibers in the annular tooth portion become same. Further, the aspect ratio of the fibers is smaller than 100, namely, the fibers are very short. Therefore, the fatigue strength of the teeth and that of the root portion of the teeth, where a strong torque is applied when the worm and worm wheel engages, was not sufficient.
An object of the present invention is to provide an electric steering apparatus improved to solve the aforementioned problems.
The electric steering apparatus according to the present invention comprises a motor for steering assists, a worm, a worm wheel having an annular tooth portion made of synthetic resin with teeth engaging with the worm and having a metal boss portion fitted in the inside of the annular tooth portion, and a steering shaft to which an output from the motor is transmitted through the worm and the worm wheel thereby to assist steering, characterized in that the annular tooth portion is made of the synthetic resin material mixed with fibers having a length with an aspect ratio between 100 and 800.
According to the present invention, the annular tooth portion is made of synthetic resin material mixed with fibers having an aspect ratio between 100 and 800, therefore, even when the longitudinal direction of the fibers is substantially parallel to the direction of the stream of the synthetic resin material in the mould, the fibers can be arranged preferably in the direction crossing the direction of the torque, which is applied to the teeth. As a result, the fatigue strength of the teeth and the root portion of the teeth can be reinforced.
When the aspect ratio of the fibers is less than 100, it is impossible to preferably arrange the fibers in the direction crossing the direction of the torque, which is applied to the teeth. As a result, only small reinforcing effect of the teeth and the root portion of the teeth can be attained by the fibers, and the fatigue strength of the teeth and the root portion of the teeth is insufficient.
Conversely, when the aspect ratios of the fibers mixed into the synthetic resin material are over 800, the fibers are cut by the injection screw, so that the aspect ratio decreases to less than 800, namely the length of the fibers with an aspect ratio over 800 cannot be realized.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.